1. Field of the Invention
The present invention relates to mobile communication systems.
2. Description of the Related Art
In this type of technical field, schemes of present and future mobile communication systems are being prepared one after another by the Third Generation Partnership Project (3GPP). In particular, as for what is to succeed a wideband code division multiple access (W-CDMA) UTRA or UTRAN system, there is a scheme called E-UTRA (Evolved Universal Terrestrial Radio Access) or E-UTRAN (Evolved Universal Terrestrial Radio Access Network). The scheme using the E-UTRA or the E-UTRAN is also called Long Term Evolution (LTE).
Therefore, various mobile communication systems co-exist at this time, and various services are provided in each of the systems. Thus, one type of service, which is provided in a certain system, may not be provided in a different system. For example, assume that a Third Generation system such as W-CDMA, CDMA2000, UMTS, or GSM, and an LTE system co-exist in the same geographical area; and the LTE system is not providing a voice service for a time period at the beginning of introduction of the system. (During the period, the LTE system is dedicated to conducting data communication.) In this case, the voice service, which is provided in the Third Generation system, is not provided in the LTE system. When a user of the LTE system desires the voice service, the user needs to communicate in the Third Generation system. When the user is visiting the LTE system, the user must be able to communicate in the Third Generation system. A scheme to make it possible to allow the user visiting the LTE system to communicate in the Third Generation system and to provide the voice service is called CSFB (Circuit Switching Fall Back), which is disclosed in Non-patent document 1, for example: